Micro-manufacturing now offers advantages for specialist processes

21st February 2013

Miniaturisation is not new, but manufacturing mechanical, electromechanical and fluidic components on a micro scale is rapidly developing into a recognisable industry in itself. Alistair Rae reports on a selection of the latest developments in this exciting field.

Over the past few years a new industry has developed to serve the growing demand for micro-manufacturing. While miniaturisation has been ongoing since the commercialisation of the transistor some 50 years ago, it is only recently that it has been viable to manufacture high volumes of mechanical and electromechanical components with features in the sub-millimetre range.

The focus for these technologies has been on products and systems for the medical/pharmaceutical, communications, sensors, defence/aerospace and mobile telephony markets.

Given the highly specialised nature of micro-manufacturing, most companies that develop production equipment or offer these services concentrate on one or a very limited number of technologies, such as micro-injection moulding, laser fabrication or micro-EDM (electro-discharge machining). Of course, there is also some interaction between the technologies, such as the need to micro-machine mould tools for micro-injection moulding.

One of the leading manufacturers of micro-injection moulding machines is Cronoplast, with its Babyplast family of benchtop machines. Materials that can be moulded include thermoplastics - such as polypropylene (PP), polyamide (PA), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polybutylene terephthalate (PBT), polyoxymethylene (POM), polyphenylene sulphide (PPS) and polyetheretherketone (PEEK) - as well as filled materials, waxes, thermoplastic elastomers, and powdered ceramics and metals for sintering. Moulded component weights range from 0.01g to 15g and multi-cavity tooling can be accommodated when production volumes warrant it. If necessary, micro-injection moulded parts can be produced with inserts.

In contrast to most conventional injection moulding machines, the Babyplast plastification system uses spheres; the temperature of the plastification cylinder is homogenous and each granule of plastic is melted by contact with hot metal. In this way, the resin is not overheated by friction. Furthermore, the compact dimensions of the plastification chamber (15cm3) ensure that the material remains at the melting temperature for only a short duration. Such measures help to ensure the material is not degraded during moulding.

While the Babyplast uses hydraulics, the Battenfeld Microsystem 50 is all-electric. This machine can mould parts with single-digit milligram weights and is available with various options to extend its functionality, such as a removal and handling module and a clean room module. A further module, which is of particular interest in the production of components for the electrical and electronics industries, enables inserts to be used.

One company that uses a Microsystem 50 is Micro Systems (UK), which offers mould design and production, as well as micro-injection moulding and micro-machining services to the medical, pharmaceutical and optical markets. An example of the type of work the company handles is the fully automated moulding and de-gating of 2.5mm diameter Fresnel lenses with a 6.5 s cycle time. The lenses are moulded in a four-cavity tool and have surface features that are between 450 and 910nm deep - though Micro Systems says it can mould smaller features if required.

For organisations that need to buy-in design expertise, Micro Engineering Solutions, which is based in the USA, offers design as well as manufacturing and assembly services. In addition to micro-injection moulding plastics, the company also undertakes micro-EDM, micro-machining, ceramic injection moulding and metal injection moulding.

Laser processing

A process that has not been discussed so far is laser cutting, or laser microstructuring, as it is sometimes known. Metafab, which is part of Cardiff University in Wales, operates a dual-beam, 157nm, 193nm excimer and a 795nm femtosecond laser, which is said to be particularly suitable for industrial microfluidic and photonic applications. Most commonly processed in this Xtreme Laser Facility (XLF) are PEEK, Teflon PTFE (polytetrafluoroethylene), Teflon AF (amorphous fluoropolymer), cyclic olefin copolymer (COC), fused silica and stainless steel. Other materials that benefit from the laser microstructuring technology include biodegradable polymers, quartz and sapphire, though Metafab claims that the XLF can machine any material. Metafab produces a wide variety of components for micro-electro-mechanical systems (MEMS), optics, microfluidics, micro-reactors and shadow masks.

Laser Micromachining Limited (LML), as its name suggests, offers laser micro-machining services, and Dr Nadeem Rizvi, the company's co-founder and general manager, also teaches a training course at FSRM (the Swiss Foundation for Research in Microtechnology) for engineers, managers and others interested in improving their understanding of laser micro-machining systems, techniques and applications. In addition to cutting and profiling, LML can also trim machined and pre-assembled devices without damaging adjacent components, micro-drill and micro-mill blind holes and pockets, and laser-mark micro-components or micro-mark conventional components.

Based in France, Steec manufactures products for applications within analytical equipment used in particle accelerators or for taking physical measurements on a molecular level, as well as components for wave guides used to capture low-level electro-magnetic energy coming from the cosmos. Interestingly, Steec uses a Yag laser for processing thin foils, tubes and micro-tubes of 3microns thickness with an accuracy of +/-5microns.

Another service available from Steec is micro-EDM using wire diameters of 30 to 100microns for profiling, or spark erosion can be used for creating complex three-dimensional cavities. Steec offers a variety of technologies for micro-drilling holes from 20microns to 3mm in diameter: micro-EDM for diameters from 40microns; micro-laser machining for diameters from 30microns; conventional micro-drilling for diameters from 60microns; and micro-punching for diameters from 20microns.

Of course, manufacturing a micro-sized component only solves half the problem; assembling such components presents a challenge that is at least as hard to address. Usually the sizes involved mean that manual assembly is impossible, hence many micro-assemblies rely on automation, with much higher accuracies being required than for conventional automated assembly systems; whereas 0.1mm would be adequate or even excessively tight for conventional assembly systems, a micro-assembly system may have to achieve accuracies in the range 0.1 to 10microns.

With micro-assembly operations, not only can it be difficult to pick up the component, but molecular forces can mean that the component is not released when the picker opens. Likewise, feeding and orientating micro components cannot be achieved using, say, a scaled-down vibratory bowl feeder. One technology that offers potential here is vision-guided robotics: a vision system can be used to identify individual components and their orientation, enabling a robot to be directed to pick up the component and reorientate it so that it can be positioned correctly.

Even when the components have been successfully brought together, there is the question of how they are attached to each other. Threaded fasteners are not generally suitable, but adhesive bonding, welding and cold deformation processes (akin to riveting) are feasible. Another technique that is currently being investigates is the use of folding - similar to the paper-folding art of origami - whereby a blank is formed into a three-dimensional component.

Finally, the assembly must be inspected to check that all components are present and correctly positioned. While manual inspection using a microscope is possible, machine vision systems are more likely to be employed for this task. Furthermore, it may be better to use automated in-process inspection at one or more intermediate points within the assembly operation.

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